This invention relates to tone detecting systems and, more particularly, to an improved programmable tone detector in digital form.
Tone detection arrangements of one sort or another are of course known in the art, as applied especially in radio wave communication systems. There, for example, each of a plurality of transceivers may be assigned its own distinct tone, or tone sequence code. Communication to an individual transceiver is accomplished by initially broadcasting the appropriate transceiver code. Tone detection circuitry within the transceiver responds by signalling that a broadcast to that transceiver is desired.
Tone detectors necessarily employ bandpass filters. If a plurality of code tones are located within a given frequency spectrum, the bandpass of each filter must be narrow enough to pass the selected tone. Moreover, the bandpass filter characteristics must be stable both as to temperature and time so as to assure proper operation of the communication system. Conventional tone detector filters include mechanical reed filters, programmable active filters, and more recently phase lock loop detectors.
A major disadvantage to all such prior art systems, however, is their need for discrete capacitors, resistors, and inductors. Not only do these discrete components consume precious space, but they also are subject to temperature effects and aging. Further, the handling of numerous discrete components increases manufacturing costs and reliability problems.
The development of large scale integration techniques, especially as applied to digital signal processing, has offered additional options to the circuit designer. Literally hundreds of active devices may be fabricated and interconnected on monolithic, macroscopic chips. Whereas, formerly, the digital equivalent of an analog circuit might encompass scores of discrete active devices, the above mentioned integration makes many of the digital counterparts increasingly more appealing. In addition, by time multiplexing techniques a single digital circuit can process a plurality of signal lines, thus avoiding circuit duplication. Further, digital designs can achieve a higher precision, and maintain the precision over wider temperature ranges and time intervals, than analog circuits. Moreover, by using digital signal processing the designer can drastically reduce, or eliminate, the need for external discrete components, thus realizing savings in cost and space.